Ectasia following small-incision lenticule extraction (SMILE): a review of the literature

Abstract

Purpose: Four cases of corneal ectasia after small-incision lenticule extraction (SMILE) have been reported. In this review, we provide an overview of the published literature on corneal ectasia after SMILE and risk factors associated with this complication.

Methods: Case reports were identified by a search of seven electronic databases for pertinent heading terms between 2011 and July 2017. We identified patient characteristics and surgical details including preoperative topography, central corneal thickness, and anterior keratometry (Km). Residual stromal bed (RSB) values not reported were computed using VisuMax ReLEx SMILE software Version 2.10.10. Preoperative ectasia risk was measured using the Randleman Ectasia Risk Score System (ERSS). Percent tissue alteration was calculated for each patient as described by Santhiago et al.

Results: Seven eyes of four patients developed corneal ectasia post SMILE. Two patients had abnormal topography in both eyes. One patient had abnormal topography in one eye. Only one patient was noted to have normal topography in both eyes and later developed ectasia in one eye in the absence of any known risk factors. The mean Randleman ectasia risk score was 4±3 (range: 1-8). The mean calculated percent tissue altered (PTA) was 38%±6% (range: 30%-47%).

Conclusion: A majority of reported ectasia cases occurred in patients with subclinical keratoconus. These conditions may be exacerbated by SMILE and should be considered absolute contraindications to the procedure. Three patients were identified to have high risk based on the ERSS, and one patient exhibited a PTA ≥40%. We formulated a modification to the current calculation of PTA that takes into account the differences in tissue altered between SMILE and laser in situ keratomileusis (LASIK). More studies are needed to fully quantify the risk of ectasia. For now, we propose adopting the same exclusion criteria used for LASIK in the SMILE procedure until more specific metrics have been validated.

Keywords: Ectasia Risk Score System; SMILE; ectasia; keratoconus; percent tissue altered; small-incision lenticule extraction.

Figure 1

Schematic representation of SMILE incision and LASIK flap incision with respect to the corneal limbus. Notes: SMILE cap circumference and small-incision length exhibited using a conventional diameter of 7.5 mm and hinge angle of 90°. LASIK flap circumference and flap incision length exhibited using a flap diameter of 8.75 mm and hinge angle of 55°. Abbreviations: SMILE, small-incision lenticule extraction; LASIK, laser in situ keratomileusis.